Dictionary.net

(DSU or "data service unit") A device used indigital transmission for connecting a CSU (Channel ServiceUnit) to Data Terminal Equipment (a terminal or computer),in the same way that a modem is used for connection to ananalogue medium.A DSU provides a standard interface to a user's terminal whichis compatible with modems and handles such functions assignal translation, regeneration, reformatting, and timing.The transmitting portion of the DSU processeses the customers'signal into bipolar pulses suitable for transmission over thedigital facility. The receiving portion of the DSU is usedboth to extract timing information and to regenerate mark andspace information from the received bipolar signal.